Without limiting the scope of the invention, its background will be described primarily with reference to offshore risers used in sub-sea production wells as an example. Submerged cylindrically-shaped objects, such as risers, spars, or other elongated cylindrical structures used for under-sea oil or gas production, pumping, or loading are often exposed to relative movement of a body of fluid, particularly moving sea currents. Such elongated cylindrical structures are common in offshore petroleum exploration, production and transportation. Sometimes such elongated cylindrical structures extend from the surface to hundreds of meters below the surface, as in the case of spar platforms for production. Sometimes the cylindrical structures extend from the seabed thousands of meters upward toward the surface and into sea currents, as in offshore production risers, loading and unloading risers or hybrid risers for petrochemical production or transport. Cylindrical riser structures may support on their exterior or encase one or more pipelines or risers extending from the seabed to a drilling or production platform, to a ship or to another offshore structure or vehicle. Such risers or cylindrical riser support structures are continuously exposed to ocean currents that produce vortexes or vortices that tend to travel downstream with the current as the water moves around and past the risers. These vortices produce oscillating “lift” forces on the cylindrical structure as a result of vortex shedding and the spanwise, or lengthwise, coherence of the vortex shedding can produce substantial cumulative lift force on the elongated cylindrical structure. The effect is particularly adverse in the case of a cylindrical riser support column extending several hundreds of meters in the path of the current.
The lift forces due to vortex shedding act generally normal to the axis of the cylindrical structure and flow direction. As a vortex is produced and then separated in a “sheet” from the cylindrical surface along the length or span of the cylinder exposed to the current, the lift force can be significant and destructive. The vortices are swirling currents that repeatedly shed from the cylinder, sometimes called “Von Karman Vortex Sheets” and produce vortex-induced vibration. The vibratory movement or vortex-induced vibration (VIV) Von Karman Vortex caused by the repeated sheet separation from the cylinder is sometimes called “Aeolian Vibration.” This vortex-induced vibration creates cyclic stresses on the cylindrical structure that may be too small to cause immediate fracture, but upon constant repetition may weaken or damage the riser through material fatigue or stress-induced fracture. In certain relatively common current situations, a resonant vibration can be created, causing repetitive forces in phase with the vibratory motion that can overstress the cylindrical structure to potentially catastrophic failure.
In the past, fins protruding from the peripheral surface of the cylinders exposed to the current or other fluid movement, as in production riser situations, were used to reduce the adverse effect of such vortex formation and vortex sheet shedding. For example, helically-arranged vortex-shedding ribs, or strakes, have been designed to be installed on submerged risers exposed to ocean currents. In one prior device, such strakes are to be incorporated as components of a flexible wrap or panel to be disposed about and secured to the submerged riser. Typically the strakes are to be clamped to the riser prior to its being submerged. Such strakes could be formed by pairs of clamping flanges mounted along the adjacent edges of elongated parallelogram-shaped wrap segments. The wrap segments could be positioned side-by-side, twisting around the outer surface of the riser, and then bolted to engage at the clamp flanges, forming a helical strake extending in a spiral around and along the length of the cylindrical structure that will be exposed to moving current.
In another design, one or more ribs or strakes could be attached vertically or diagonally on a flat, rectangular panel of flexible wrapping material. The wrapping material would be dimensioned to encircle, by itself, an elongated segment of a single riser, piling, pipe or other cylindrical object. Clamping flanges were to be mounted along opposed vertical edges of the rectangular panel. The clamping flanges were to be brought together and clamped, thereby stretching the panel to wrap securely around and frictionally embrace the outer surface of the riser. A plurality of such wrapped panels with ribs or strakes were to be clamped in deployed positions, along the length of the cylindrical structure such that the strakes were aligned at either end of adjacent panels in a helical configuration encircling the wrapped riser structure.
It is difficult to transport, handle and install a cylindrical riser support structure having protruding strakes. Further, it has been found that installation underwater at the riser site is extremely difficult and usually impractical. It has been found that fabrication of a cylindrical riser structure with a protruding strake of a prior design is costly. Additionally, it has been found that the protruding strake on a cylindrical riser support structure increases the viscous drag of the water against the riser assembly, thereby risking greater stress and requiring increased size and strength for the riser support design.
In certain riser installations, a polymeric coating and, in particular, a polymeric foam layer is applied to the exterior surface of the risers and the riser support cylinder to provide protection from the undersea environment and advantageously to provide buoyancy to the assembly. The riser itself may be composed of a metal or a composite material. The riser support structure is normally a metal support cylinder with the metal or composite cylindrical riser pipe lines and polymeric foam coating material attached to the surface of the metal cylinder to facilitate maintaining the riser and support structure in an upright position by reducing the combined mass density (i.e., by adding buoyancy). It has been found that securing strakes, of any prior known design, to the exterior of a layer of polymeric foam is difficult. For example, clamping of strakes to the polymeric surface often fails due to insufficient compression strength of the foam. Particularly, in the case of a polymeric foam coating or bundle on the riser or riser support cylinder, clamping tension may not be sufficient to maintain the strakes in a secure position. Excessive clamping tension can significantly reduce the buoyancy by crushing the foam layer.
A need has therefore arisen for a device, mechanism and method to reduce, resist or suppress vortex induced vibration (VIV), or the effect of VIV on submergible cylinders such as risers and riser support columns, without requiring the attachment of a protruding strake. A need has also arisen for a submergible riser assembly with a VIV reduction mechanism attached that is easy to transport, easy to handle and easy to install and that is not costly to fabricate. In addition, a need has arisen for such a VIV reduction mechanism for fluid immerse cylindrical structures and assemblies, including submergible riser assemblies that does not significantly increase the viscous drag of moving fluid or moving water against the immersed cylinder or submerged riser assembly.